The present invention is directed to a lithium(Li)-air battery having high energy density and cyclability.
Lithium ion technology has dominated the market as an energy source for small electronic devices and even plug-in hybrid and electric vehicles. However, Li-ion batteries provide insufficient energy density to act as a power source for future generations of electric vehicles, which would require the use of higher energy density power sources.
Metal-air batteries have been under investigation as an advanced generation of high energy density, energy sources which have the potential to power vehicles for distances comparable to present fossil-fuel based combustion engines. In a metal-air battery, the metal of the anode is oxidized and the resulting cation travels to the cathode containing a porous matrix of a material such as carbon, for example, where oxygen is reduced and the reduction product combines with the metal cation to form the oxide, superoxide or peroxide discharge product. Upon charge, this process is ideally reversed. Metal-air batteries are recognized to have potential advantages over metal-ion batteries because the cathode material, oxygen, may be obtained from the atmosphere, and thus the capacity of the battery would in theory be limited by the anode metal supply. Thus, an oxygen gas supply would be supplied continuously from outside the battery and battery capacity and voltage would depend upon the oxygen reducing properties of the cathode and the chemical nature of the discharge product formed.
For example, Li-air batteries use inexhaustible oxygen from outside (i.e. air) instead of storing an oxidizer inside. Therefore, a Li-air battery has much higher energy density when compared with a conventional Li-ion battery and has potential for application in the field of long-range electric vehicles. However, unsolved fundamental problems such as poor oxygen redox kinetics at the cathode and deleterious volume and morphology changes at the negative electrode greatly limit the practical application of Li-air batteries.
Li-air batteries have the potential to supply 5-10 times greater energy density than conventional lithium ion batteries and have attracted much interest and development attention as a post Li-ion battery technology. For example, a non-aqueous Li-air battery which forms Li2O2 as discharge product theoretically would provide 3038 Wh/kg in comparison to 600 Wh/kg for a Li-ion battery having a cathodic product of Li0.5CoO2. However, in practice, the metal air technology and specifically current non-aqueous lithium air batteries suffer many technical problems which have prevented achievement of the theoretical capacity.
The capacity of the Li-air battery is highly dependent upon the capacity of the cathode matrix to store the Li2O2 discharge product. Li2O2 is generally insoluble in conventional non-aqueous solvents employed in metal-air batteries. Therefore, upon formation at the cathode matrix, the Li2O2 precipitates and fills the surface porosity of the matrix thus preventing access to the vacant capacity of the matrix interior region. Moreover, Li2O2 is an insulator and therefore, once the surface of the matrix is coated, oxygen reduction is prevented and discharge terminated, i.e., the capacity of the battery is severely reduced in comparison to the theoretical capacity.
Cathodes containing catalysts to facilitate and enhance the oxygen reduction reaction are known and have been the subject of ongoing studies in both the academic and industrial sectors. Catalysts based on noble metals, metal oxides, metal alloys and carbon nanotubes have been described.
However, to successfully replace Li-ion cells, Li-air batteries having improved cycle life, improved coulombic efficiency and increased energy density must be developed.
Key problems that must be addressed to achieve success with Li-air battery technology include electrolyte stability, deposition of Li2O2 and passivation of catalyst surfaces.
The products of oxygen reduction at the cathode of a Li-air battery include lithium peroxide (Li2O2) which is generally insoluble in the electrolyte medium and therefore, precipitates onto available surfaces of the matrix and catalyst. Once formed the Li2O2 is difficult to decompose during charging of the battery and therefore Li2O2 deposits build on the catalytic surfaces and adversely retard catalytic performance.
This problem must be addressed and overcome to develop and produce an efficient, safe, cost effective, high capacity Li-air battery useful especially for powering vehicles to distances at least equal to or competitive with current fossil-fuel systems.